This invention relates to a sensor for sensing acceleration. More particularly, the invention relates to an acceleration sensor suitable for use in sensing deceleration in conjunction with actuation of a tensionless release mechanism of a belt retractor in a vehicle seat belt system, i.e., a mechanism that releases a tension relief mechanism of the retractor and restores winding force to the belt reel.
When a vehicle such as an automotive vehicle accelerates, decelerates or turns, acceleration (hereinafter sometimes referred to as "G") conforming to such motion acts upon the vehicle. In order to sense G and take suitable measures to protect the body of a passenger in the vehicle, a G sensor is used as means for actuating a passenger protective apparatus.
By way of example, a sensor of this type in an automotive vehicle is used for actuating a tensionless release mechanism of a retractor that winds up the seat belt in a seat belt system.
FIG. 8 is a side view schematically illustrating the construction of a previously proposed G sensor. The sensor includes a pendulum 02 having a body or mass 01, a switch 03, a signal line 04 and a sensor frame 05. The pendulum 02 is supported on the sensor frame 05 by a shaft 06 to swing freely in the direction of arrow A in FIG. 8 when acceleration acts upon the sensor, or, more precisely, the pendulum. Owing to swinging of the pendulum 02, the switch 03 is opened and produces an output signal on the signal line 04. The sensor is further provided with a spring 07 which biases the pendulum 02 in the direction of arrow B in FIG. 8 for the purpose of closing the switch 03, and a buffer 08 which absorbs the impact of the body 01 against the sensor frame 05.
Since a sensor of this kind is adapted to output a signal in response to a predetermined G, an output in response to a value of G which differs depending upon the prevailing conditions cannot be obtained.
With a tensionless release mechanism of a retractor, there is a need for operation which conforms to the psychological state of the passenger. In particular, there are cases where it is desired that a G that results in actuation of the release mechanism be made to have different values at high velocity and low velocity travel of the vehicle.
When it is attempted to realize such an operating system using the already proposed sensor described above, a complicated system is required in which a multiplicity of sensors are used and a signal circuit is provided for selecting an output from one of the sensors in dependence upon travelling velocity. Such a system involves problems in terms of reliability and cost.